1. Field of the Invention
This invention relates to new organometallic compounds formed by direct metallation of weakly acidic organic compounds with barium, strontium and/or calcium metal in the presence of an aprotic polar solvent. The metallation reaction involves a metal-hydrogen exchange to create compounds in which metal is joined to an organic radical by an ionic carbon to metal bond. These compounds are highly active as polymerization catalysts in initiating anionic growth-type polymerization of various monomers, e.g., alkylene oxides, vinyl compounds or the like, to form homopolymers and copolymers.
2. Description of the Prior Art
It has long been known that organometallic compounds in which metal is joined to an organic radical through a carbon to metal bond, may be prepared by a variety of methods. The Grignard reagents constitute a historic example of this type of compound. Tetraethyl lead and aluminum alkyl compounds are other examples of organometallic compounds having carbon to metal bonds which can be prepared quite readily in good yield and purity. Actually, the ready availability of aluminum alkyls has prompted their use as starting materials for formation of more complex organometallic compounds (see U.S. Pat. No. 3,360,537). A metal alkyl such as zinc diethyl has been used in metal interchange reaction for the preparation of organometallic compounds of strontium (see Gilman et al., J.A.C.S., 65, 268-1943). Alkaline earth organometallic compounds formed in this manner were found to have low solubility in organic solvents such as benzene (see Gilman et al., J.A.C.S., 67, 520-1945).
It is known that some hydrocarbons possess carbon-hydrogen groups which exhibit ionic-like properties forming groups which are referred to as carbanions. Such hydrocarbons exhibit an acidic like property which is stronger in the case of some hydrocarbons than with others. This has resulted in the development of an acidity scale by which the relative ease with which the hydrocarbon forms a carbanion is characterized by a pKa value on a McEwen - Streitwieser - Applequist -Dessy (M.S.A.D.) scale (see "Fundamentals of Carbanion Chemistry" by Cram, page 19, published 1965 by Academic Press). On this scale, the lower the pKa value for a hydrocarbon, the more acidic it is and the greater is the ease with which the hydrocarbon will form a carbanion.
Due to the highly basic nature and strong reactivity of alkali metals, formation of organometallic compounds of alkali metals with acidic type organic compounds can be readily accomplished. These can then be used in metal-hydrogen interchange reactions to form the corresponding organometallic compounds of other less reactive or less basic metals (see U.S. Pat. No. 3,450,728). Where it is desired to form organometallic compounds of less basic metals, e.g., alkaline earth metals, a technique for indirect metallation has been developed. Thus, by conducting a reaction of a metal with an acidic hydrocarbon in liquid ammonia as a reaction medium, calcium hexammoniate which forms between calcium and the liquid ammonia will react with an acidic hydrocarbon to form an organometallic compound (see U.S. Pat. No. 3,365,404).
Most aliphatic hydrocarbons do not have sufficient acidity to permit preparation of alkyl organometallic compounds from alkaline earth metals by direct metallation. Formation of such compounds by reaction of methyl iodide with barium, strontium or calcium has been conducted using pyridine as a reaction medium (see J.A.C.S., 80, 5324-1958).
Cyclopentadiene is a hydrocarbon exhibiting relatively high acidity, i.e., it possesses a pKa value on the MSAD scale of 15. However, the hydrocarbon is not sufficiently "acid" to react with strong bases such as sodium hydroxide although it will react directly with sodium metal. In order to produce the calcium adduct of cyclopentadiene, the hydrocarbon has been reacted with calcium carbide using liquid ammonia or certain amines as accelerators for the reaction (see U.S. Pat. No. 2,835,712). A similar technique has been used in the formation of other calcium metallo-organic compounds such as calcium adducts with triphenylmethane and fluorene (see U.S. Pat. No. 3,365,404).
Organometallic compounds are known to be useful as catalysts for polymerization of a variety of polymerizable monomers. Actually, such adducts of metals vary in catalytic activity depending upon the metal present in the compound, the method of preparation of the adduct and its possible association in complexes with other materials. Such catalytic materials are of great commercial significance because they provide unique catalytic procedures, e.g., stereospecific polymerizations of the Ziegler-Natta type. Al, Be, Ca, Mg and Zn amide-alcoholate compounds have, for example, been used to polymerize monomeric cyclic carbonates (see U.S. Pat. No. 3,301,824). The adduct of calcium with certain acidic aryl hydrocarbons and ammonia have been used for polymerization of epoxides (see U.S. Pat. No. 3,365,404). Epoxides have also been polymerized using complexes between organometallic compounds and alcohols (see U.S. Pat. No. 3,275,598), organometallic compounds and polyoxyalkylene glycols (see U.S. Pat. No. 3,427,259) or complexes of organometallic compounds with polar solvents (see U.S. Pat. No. 3,337,475). Also, barium and other group II-A metals have been treated with certain compounds like naphthalene to form radical ions or anions and used in the polymerization of isoprene, styrene and so forth: These barium compounds react as follows: ##STR1## so that metallation of the naphthalene does not occur. Rather, complexes are formed by a transfer of electrons from the metal to the naphthalene which has a high electron affinity. See U.S. Pat. No. 3,509,067. The utility of organometallic compounds as catalysts in polymerization reactions and for other purposes would be further improved if organometallic compounds of alkaline earth metals could be created by less complicated procedures than available heretofore that would produce the organometallics in good yield and in a form in which the compound exhibits high carbanion content and catalytic activity.